This application relates to integrated circuits, and more particularly to a MOSFET with an integrated controller driver circuit to function as a rectifier.
When using the terms “integrated” or “integrated circuit”, it is to be understood that an integrated circuit is the preferred method of construction, but the teachings of this invention would be equally applicable to a circuit made partly of integrated circuits or entirely of discrete components.
U.S. Pat. No. 6,271,712, Alan Ball, issued Aug. 7, 2001 and entitled “Synchronous Rectifier and Method of Operation” shows an integrated synchronous rectifier intended for the same applications as the synthetic rectifier of this invention. In distinction to the present invention, Ball switches reactively after sensing zero crossing of the current or the voltage, and does not have or suggest any predictive mechanism.
Ball teaches a method of controlling voltage to power a synchronous rectifier controller that could be useful in the present invention. However, it is a variant of a boot strap power supply circuit which is well known. Two of the five independent claims recite a “polarity detection circuit” to operate the synchronous rectifier MOSFET, the other three independent claims relate to the power supply circuit portion.
Problems with Prior Art Synchronous Rectifier Controllers:                1. There is no signal available that reliably predicts when a MOSFET rectifier should change state.        2. There is no way to reliably predict when switching should be initiated to achieve a specific turn-off time.        3. Those signals that are available may involve complex interfaces.        
To elaborate, turn-on and turn-off is usually initiated by sensing that a voltage has changed, somewhere. This may be through an isolation barrier from a primary controller, a complex interface. It may be a signal from a buck controller. It may be the voltage reversal on a transformer winding. In all cases, the time to ideal switching is dependant upon the load current and other factors. It would take a very complex algorithm to know the precise moment that state switching should occur. Even if the precise ideal switching time could be determined, the time that it takes to accomplish turn-on or turn-off is not well controlled.
Prior Art Predictive gate drive: A signal to switch the MOSFET is derived from a controller section and inserts a digitally derived time delay to correct for the time that it takes the current or voltage to settle down as well as the propagation time delay in the circuits in a trial and error detection scheme. The results from one interation are modified and saved from cycle to cycle in an algorithm that corrects for the cumulative time delays so that the MOSFET turns on or off at nearly the optimum time. This is a complex circuit and it does not respond rapidly to changes in the load conditions.
The proposed synthetic rectifier overcomes these limitations.
This application builds upon the technology of a provisional patent application entitled “Gate Drive Method and Apparatus for the Fast Switching of MOSFETs” Ser. No. 60/429,998, filed on 30 Nov., 2002; a provisional patent application entitled, entitled “Gate Drive Method and Local Clamp for the Fast Turn Off of MOSFETs”, Ser. No. 60/319,085 filed on 22 Jan., 2002; and a continuation in part patent application entitled “Gate Drive Method and Apparatus for Reducing Losses in the Switching of MOSFETs”, Ser. No. 10/248,438, filed on Jan. 20, 2003. These applications are incorporated herein by reference.